This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-110852, filed on Apr. 12, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a barcode scanner, and more particularly, to a barcode scanner comprising a processing circuit for an electric signal generated from a light reflected from a barcode scanned with a laser beam generated by pulse driving.
2. Description of the Related Art
A barcode scanner of a laser scanning system uses a laser diode as a light source, scans its emission output on barcode information (pattern) by a scanning mirror, reads intensity of a reflected light from a barcode by a photodetector, so that a barcode signal corresponding to the barcode information is obtained.
For example, Jpn. Pat. Appln. KOKAI Publication No. 4-177582 discloses a technology for maintaining an amount of a reflected light from a barcode label surface at a constant level by controlling a driving current of a laser diode in accordance with the amount of a laser beam reflected from the barcode label surface.
However, an output signal obtained in accordance with the amount of the laser reflected beam contains a DC offset component caused by an ambient light such as sunlight in addition to a barcode signal. Thus, as in the case of a black part of a barcode, the DC offset component is relatively enlarged in a place where a reflected light is small and amplitude of a received signal becomes small, which reduces reading accuracy of the barcode.
Further, while the DC offset component and a noise signal level contained in an input signal is almost constant, the intensity of the reflected light from the barcode is inversely proportional to a distance between the barcode and the laser light source, i.e., a square of a distance between the barcode and the scanner, and thus detected signal intensity is lower as a distance is larger.
Therefore, it is necessary to set a signal gain higher as a distance is larger, but an offset of a detected signal output due to noise or the like is also enlarged. While there is a demand for increasing the readable distance between the barcode and the scanner, a problem of difficulty of making the maximum readable distance long occurs.
There is also a demand for a faster barcode reading speed. However, if a barcode scanning speed by a laser beam is increased, a problem that amplitude of a detected signal becomes smaller occurs. To deal with this problem, for example there is an invention disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-346184.
According to this conventional technology, it is possible to obtain a binarized signal where the DC offset due to the disturbance light and the influence of the noise component are removed. However, since this conventional technology does not deal with fluctuation in a frequency component of a barcode detected signal inputted to a binarization circuit caused by distance between the barcode and the scanner or thickness of bars and blanks in barcode pattern, barcode information may not be read correctly. Consequently, as described above, while there is a demand for an increase of a readable distance between the barcode and the scanner, the readable distance cannot be increased, thereby disabling the performance improvement of the barcode scanner.
Generally, a reflected light from a barcode which is an object of the barcode scanner is weaker as a distance between the barcode and the scanner is longer, and a high frequency component contained in the barcode detected signal is increased. The frequency component of the reflected light from the barcode depends not only on the distance between the barcode and the scanner but also on a pattern interval (thickness of bars and blanks in barcode pattern) of a barcode print, and varies depending on an angular velocity of a light source for scanning on the object.
Frequency characteristics of the conventional binarization circuit of the aforementioned constitution are fixed, and frequency characteristics of its signal processing are fixed. Thus, if a frequency component of an inputted barcode signal fluctuates due to a distance between the barcode and the scanner or thickness of bars and blanks in a barcode pattern, there is a possibility of a problem of deviation of signal processing characteristics from optimal conditions to deal with fluctuation in the frequency component.
A barcode scanner according to one aspect of the present invention comprises: a first oscillator to generate a pulse signal of a predetermined frequency; a laser driving circuit to pulse-drive a laser by the pulse signal generated from the first oscillator; and a signal processing section which detects a reflected light signal from a barcode scanned by an emission output of the laser to generate a binarized signal corresponding to the barcode, wherein the signal processing section includes: a preamplifier to convert, into a voltage signal, a current-converted barcode signal outputted from a photodetector which detects the reflected light from the barcode scanned by the emission output of the laser, and to supply an amplified voltage signal; a high-pass filter to remove a direct current component contained in the amplified voltage signal from the preamplifier; a variable gain amplifier which controls an amplification gain of an output signal of the high-pass filter, thereby carrying out automatic gain control so as to set an output level almost constant, a synchronous detector to detect a pulse signal contained in an output signal of the variable gain amplifier in synchronization with the pulse signal generated from the first oscillator, and a binarization circuit to binarize an output signal of the synchronous detector.